1. Field of the Invention
The present invention relates to digital modulation apparatus and techniques, and more particularly, to frequency-shift keying (FSK) digital modulation.
2. Description of the Related Art
Digital modulation is the process of preparing digital data signals for transmission, such as transmission in wireless communications. For long distance transmission of carrier signals, the digital data is typically used to modulate the carrier signal before transmission. The result is referred to as amplitude-shift keying (ASK), phase-shift keying (PSK), or frequency-shift keying (FSK) modulation if the amplitude, phase, or frequency, respectively, of the carrier signal is modulated in accordance with the digital data.
With respect to FSK modulation, the digital data signals shift the output frequency of the carrier between predetermined values, making FSK a form of frequency modulation. A binary 0 is typically represented by a first frequency f0, and a binary 1 is typically represented by a second frequency f1. The peak frequency deviation Δf is generally equal to (f1−f0)/2, and the modulation index h is equal to Δf/R, where R is equal to the symbol rate 1/T. The variable T represents the bit interval of the input digital data (or message) signal.
If the digital signals input to the modulator assume one of only two possible values, the communication system is referred to as binary. For example, binary PSK modulation is referred to as BPSK. If one of M>2 possible values is available the system is referred to as M-ary. An important M-ary modulation scheme, quadriphase-shift keying (QPSK), is often employed in situations in which bandwidth efficiency is a consideration. Systems having M>4 are often referred to as multilevel, e.g., multilevel FSK (MFSK) and multilevel PSK (MPSK).
A digital system is referred to as being “coherent” if a local reference is available for demodulation, where the local reference is in phase with the transmitted carrier. Otherwise, the system is referred to as being “noncoherent”. While the performance characteristics of coherent systems are generally better than noncoherent systems, noncoherent systems have the advantage of simplicity of implementation.
Conventional analog FSK modulators typically attempt to directly modulate the voltage controlled oscillator (VCO) component contained in an analog phase locked loop (PLL). Although this technique has the advantage that the modulated carrier is generated with a single device, the technique has several inherent problems. Specifically, the frequency deviation and zero-crossing aspects of the waveform are difficult to control because of the behavior of the PLL. This is because the digital data going into the VCO appears to be a disturbance or phase error and the PLL tries to correct it to obtain a pure carrier. Furthermore, random digital data often results in a frequency offset. Some conventional designs attempt to bypass the PLL by opening the loop during modulation, but this technique introduces problems with excessive pulling and carrier frequency drift.
Direct digital synthesis (DDS) devices overcome many of the performance disadvantages associated with the prior art analog modulators. Specifically, DDS devices are capable of digitally creating sine waves. In addition, DDS devices are typically capable of implementing several types of modulation schemes. DDS designs allow designers to control an output sine wave's frequency and phase to extremely high resolution. Disadvantageously, the DDS devices of the prior art are typically complex and have significant power and cost requirements. Conventional DDS devices typically include frequency registers for use in implementing FSK, phase registers for use in implementing PSK and QPSK, a digital phase accumulator, multiplexers for interconnecting the registers and the digital phase accumulator, a sine wave ROM look-up table (LUT), and a digital-to-analog converter (DAC). The inclusion of frequency registers, phase registers, and multiplexers cause conventional DDS devices to be relatively complex circuits typically requiring a significant amount of precious integrated circuit (IC) silicon area. Furthermore, conventional sine wave LUTs typically requires relatively large ROMs. All of these factors cause conventional DDS design approaches to produce relatively complex, large and expensive modulators.
A digital modulation technique that is currently being used in wireless communication systems is Gaussian filtered FSK (GFSK). In accordance with this technique, the digital baseband signal is processed by a premodulation Gaussian shaping filter. Specifically, the digital data waveform (with rectangular binary 1 and binary 0 pulses) is first filtered by a low-pass filter having a Gaussian-shaped frequency response. This Gaussian filtered data waveform is then input to the frequency modulator to generate a GFSK signal.
One advantage of GFSK as compared to FSK modulation techniques is that the rectangular input pulses used in FSK modulation result in piecewise linear phase changes which tend to widen the output spectrum. Smoothing these phase changes, however, using a filter having a Gaussian impulse response circumvents this problem. The Gaussian-shaped impulse response filter generates a signal having lower side lobes and a narrower 20 dB bandwidth than a rectangular pulse. Such filtering removes the higher frequency components in the digital data waveform and, therefore, yields a more compact spectrum. Other advantages of GFSK modulation include: the narrow power spectrum with a narrow main lobe and low spectral tails maintain the adjacent channel interference at low levels and thereby achieve increased spectral efficiency; GFSK can be demodulated using noncoherent systems which results in low cost GFSK receivers; and GFSK maintains a constant envelope waveform.
The key parameter of GFSK modulation in controlling both bandwidth and intersymbol interference (ISI) is the 3 dB down filter-bandwidth B multiplied by the bit interval T (commonly referred to as the “BT product”). The BT product is known as the normalized bandwidth. When BT<1, the frequency shaping pulse has a duration that is greater than T causing ISI to be introduced. As BT decreases, the induced ISI is increased. Thus, whereas a smaller value of BT results in a more compact power density spectrum, the induced ISI will degrade the bit-error-rate (BER) performance. Hence, there are tradeoffs in the selection of the BT product.
An example of one wireless communication standard that requires GFSK modulation is the so-called “Bluetooth” standard. Bluetooth requires that data be transmitted at a symbol rate of 1 Msps (Megasymbols per second). A Gaussian-shaped, binary FSK modulation is applied with a BT product of 0.5. A binary one is represented by a positive frequency deviation. A binary zero is represented by a negative frequency deviation. The modulation index is specified to be 0.28-0.35, and the maximum frequency deviation is specified to be between 140 kHz and 175 kHz.
One conventional means for implementing a fully digital GFSK modulator is accomplished by combining a digital Gaussian filter with a DDS device. Such devices, however, suffer from the disadvantages of being relatively complex, large, power consuming and expensive devices. This is due not only to the complexity of the DDS devices as described above, but also to the complexity of conventional digital filters. Specifically, digital filters typically perform one or more mathematical functions on an input digital data stream. Digital filtering involves a computational process or algorithm by which a sampled input signal or sequence of input numbers is transformed into a second sequence of output numbers. The computational process typically involves the summation of scaled samples of the input signal. Characteristics of the digital filters can be altered under software control, which adds to their overall flexibility. As such, conventional digital filters typically include several adders, multipliers, and accumulators. The inclusion of these components increases the complexity, size and cost of the digital filters.
The size and power requirements for wireless communication devices continues to decrease as more and more of the components of such devices are implemented in digital circuitry. Therefore, a need exists for a relatively small, simple, accurate and power conserving fully digital GFSK modulator design that overcomes the disadvantages of the prior art conventional GFSK devices.